I was struggling with this one, because I had a line like this:
<%= stylesheet_link_tag 'application',
media: 'all', 'data-turbolinks-track' => true %>
It's hard to tell where to put your link at a first sight.
That you wanted to do is to place your stylesheet link AFTER 'application', divide it by commas. Like this:
<%= stylesheet_link_tag 'application',
'https://fonts.googleapis.com/css?family=Open+Sans:400&subset=latin',
media: 'all', 'data-turbolinks-track' => true %>
Then use it in your stylesheet as usual with font-family property.
Of course, we can @import it in a SASS, or do it as usual with a link tag, but that's just an another way and it's should be mentioned here.